Rich pickings: A diver catches tuna in a "fattening pen" near Port Lincoln, South Australia

There's a lot about tuna that Hagen Stehr still doesn't understand, but he's sure of one thing. "When I was young I could make love anywhere  in the street, on the boat, in the park, anywhere," booms the 66-year-old fishing magnate from Port Lincoln, South Australia. "Later in life, you gotta have the bedroom, the light ... everything's gotta be nice and soft, the ambience gotta be right. With tuna, it's no different. Everything's gotta be right."

For the endangered southern bluefin tuna, prized in Japan for its texture and taste as sushi...